1. Field of the Invention
The present invention relates to a ferroelectric oxide structure, such as a ferroelectric element, and a liquid discharge apparatus using the ferroelectric oxide structure. Further, the present invention relates to a method for producing the ferroelectric oxide structure.
2. Description of the Related Art
A piezoelectric element including a piezoelectric body and electrodes for applying an electric field to the piezoelectric body is used as a piezoelectric actuator or the like, which is mounted on an inkjet-type recording head, an atomic force microscope (AFM), a camera module of a cellular phone, an ultrasonic wave device or the like. The piezoelectric body has piezoelectric properties, in other words, expands or contracts as the strength of the electric field applied to the piezoelectric body changes. In recent years, a need for reducing the sizes of various kinds of electronic devices and a need for highly densely integrating various kinds of electronic devices have increased. Therefore, some attempts have been made to reduce the thicknesses of the electronic devices by structuring the electronic devices as thin-film deposition structures. Further, a structure including a piezoelectric thin-film is used in a piezoelectric element. In such a structure, it is desirable that thickness of the piezoelectric thin-film is greater than or equal to 200 nm to obtain efficient piezoelectric properties. Further, it is more desirable that the thickness of the piezoelectric thin-film is greater than or equal to 500 nm.
As the piezoelectric material, a perovskite-type oxide, such as lead titanate zirconate (PZT), is widely used. Such a piezoelectric material is a ferroelectric material that spontaneously polarizes when no electric field is applied to the piezoelectric material.
Lead-based perovskite-type oxides, such as PZT, are most widely used among piezoelectric materials. The lead-based perovskite-type oxides are known to have a large ordinary electric-field-induced piezoelectric strain, in which the piezoelectric material expands or contracts in the direction of the application of the electric field as the strength of the applied electric field changes.
Recently, there is growing concern about loads on the environment, and restriction on the use of lead is started also in material fields, such as RoHs regulation in Europe. However, with regard to piezoelectric materials, the piezoelectric properties of lead-free piezoelectric materials are insufficient, compared with the piezoelectric properties of lead-based piezoelectric materials. Therefore, the piezoelectric materials are excluded from the regulations. Hence, a lead-free piezoelectric material that has excellent piezoelectric properties similar to those of the lead-based piezoelectric material needs to be developed.
In lead-free piezoelectric materials, a strain displacement amount is limited if only the aforementioned ordinary electric-field-induced piezoelectric strain is utilized. Therefore, a piezoelectric element utilizing reversible non-180-degree domain rotation, such as 90-degree domain rotation, has been proposed. In such a piezoelectric element, when the piezoelectric material has a tetragonal crystal system, it is possible to obtain both of a piezoelectric strain that is obtained by 90-degree domain rotation of a-domains to c-domains and an ordinary electric-field-induced piezoelectric strain obtained after the domain rotation. In the a-domains, a-axes are oriented in the direction of application of the electric field, and in the c-domains, c-axes are oriented in the direction of the application of the electric field. The a-domains rotate to the c-domains by application of the electric field.
L. X. Zhang and X. Ren, “In situ observation of reversible domain switching in aged Mn-doped BaTiO3 single crystals”, Physical Review B 71, pp. 174108-1-174108-8, 2005 (Non-Patent Literature 1) discloses a piezoelectric material in which movable point defects are arranged in a single crystal of barium titanate having c-axis orientation ((001) orientation) in such a manner that the short-range order symmetry of the point defects becomes the same as the crystalline symmetry of a ferroelectric phase. Further, Non-Patent Literature 1 has reported that in this material, a tetragonal crystal phase of a-domain structure ((100) orientation) in which the spontaneous polarization axis and the direction of the application of the electric field are shifted by 90 degrees is formed, and that reversible 90-degree domain rotation of this domain occurs. However, in the piezoelectric material disclosed in Non-Patent Literature 1, a-domains are present in c-domains in a mixed state. Therefore, the ratio of the a-domains is low, and a sufficient domain-rotation effect is not achieved. The domain-rotation effect of the piezoelectric material that has a tetragonal crystal system is most efficiently achieved when the piezoelectric material has a-axis single-orientation ((100) single orientation).
Further, Japanese Unexamined Patent Publication No. 7 (1995)-300397 (Patent Literature 1) discloses a ferroelectric thin-film element including a ferroelectric thin-film deposited on a substrate. In Patent Literature 1, the average thermal expansion coefficient of the substrate from room temperature to the deposition temperature of the ferroelectric thin-film is less than or equal to 50×10−7° C.−1, and the ferroelectric thin-film is strongly oriented in <100> direction.
Patent Literature 1 (Example 1 and the like) describes, in Example 1, that lead lanthanide titanate thin-films are deposited on substrates that have different average thermal expansion coefficients from each other by using a high-frequency magnetron method, and that the difference in the average thermal expansion coefficients of the substrates influences the crystal orientation of each of the thin-films (thickness is fpm) deposited on the substrates (FIG. 1). Further, Patent Literature 1 describes that when the average thermal expansion coefficient of the substrate is less than or equal to 50×10−7° C.−1, the thin-film is strongly oriented in <100> direction. However, in the XRD illustrated in FIG. 1 of Patent Literature 1, an orientation peak in <001> direction is observed in each of (A) through (C), which are judged to be strongly oriented in <100> direction. Therefore, a single-orientation thin-film has not been obtained (the degree of orientation estimated from the XRD spectrum is approximately 80%).